1. Field of the Invention
The present invention relates to a method of and a computer system for executing area-division and compression of a document image.
2. Description of Prior Art
Generally, in an image processing system for digital printing, a document image is read using a scanner or the like to temporarily store image data corresponding to the document image onto a memory, and then an image is printed based on the image data stored on the memory. In order to efficiently utilize a memory limited in storage capacity, or reduce the storage capacity of a memory to be installed in the image processing system, or increase the transmission rate of the image data, the image processing system is configured such that the image data is stored in a compressed form, and the stored image data is expanded in the image-printing process.
As for the document image, there are various types, such as photograph and graphic, different in attribute (image characteristic), and a compression technique optimal to each image data of such document images is varied depending on their attributes. Therefore, in this type of conventional image processing system, a plurality of compression techniques are typically employed, and selectively used in conformity to each of the attributes of the document images.
Further, a document image on a single sheet can include a plurality of images different in attribute in a mixed manner. In such a case, if the plural types of image data of the document image are simply compressed using a single compression technique, the entire document image cannot be optimally compressed. From this point of view, there has been proposed an image compression technique in which a document image is divided into a plurality of areas based on their respective attributes, and each of the areas is individually compressed in conformity to the attribute thereof (see, for example, Japanese Laid-Open Patent Publications No. 2001-136521 and No. 10-108011).
According to the above conventional compression technique, only if a plurality of areas different in attribute are separated or independent from each other in a document image on a single sheet, respective image data of the areas can be optimally compressed. However, as the result of a process of dividing a document image into a plurality of areas based on their respective attributes, one area associated with a first attribute is likely to partly or fully overlap with another area associated with a second attribute different from the first attribute. In other words, a certain area has a plurality of attributes, and all of the plural attributes are overlappingly recognized for this area in the area division process. In such a case, the above conventional image-data compression technique cannot identify the attribute of the overlap area, and consequently cannot determine any specific compression technique to be used for the overlap area.